1. Field of the Invention
The present invention relates to an image forming apparatus and program product for image forming.
2. Description of the Related Art
In duplex printing performed by an image forming apparatus, printing is performed on, for example, a front surface of a sheet that is fed from a sheet stacking unit (referred to as “front surface printing”); the sheet is reversed in a reverse-duplex path; and printing is performed on a back surface (referred to as “back surface printing”). In the case of performing printing, the sheet needs to reach a transfer position at which a toner image formed on a photosensitive element is transferred onto the sheet. In the case of performing both front surface printing and back surface printing on one piece of sheet, the sheet positioned at the transfer position at the time of front surface printing is reversed and then reaches the transfer position again via a predetermined path (the reverse-duplex path). Thus, in the case of performing duplex printing, the sheet passes through the reverse-duplex path before back surface printing after front surface printing. Thus, compared to the case when printing is continuously performed on one surface, the conveying distance increases between two surfaces of the sheets. Thus, an arrival of the sheet to the transfer position is delayed; and the number of prints per unit time of duplex printing (referred to as productivity) may be smaller than single-sided printing.
In this regard, as a technique for improving productivity of duplex printing, there has been implemented a method of changing a printing order at the time of duplex printing. This method is referred to as interleaf control. This method is based on the premise that sheets having undergone front surface printing are scattered on the reverse-duplex path. In this method, prior to back surface printing on the sheet having undergone front surface printing, front surface printing is performed on sheets by the number of sheets which can be scattered on the reverse-duplex path. Thereafter, back surface printing on the sheets having undergone front surface printing that are scattered on the reverse-duplex path and front surface printing on a sheet newly fed from the sheet stacking unit are alternately performed. In this case, it is possible to efficiently use a time until the sheet reaches the transfer position via the reverse-duplex path after front surface printing. However, in the interleaf control, since a mechanical configuration for scattering a plurality of sheets on the reverse-duplex path is necessary, implementation conditions are restricted. For this reason, when it is difficult to perform the interleaf control, duplex printing is performed in a conventional printing order such that front surface printing is performed one sheet; the sheet is conveyed to the transfer position via the reverse-duplex path and subjected to duplex printing; and then front surface printing and back surface printing are performed on the next sheet in the same manner.
However, in conventional duplex printing, regardless of the implementation of interleaf control, timing for newly feeding a new front surface printing target sheet (referred to as “front surface printing sheet”) from the sheet stacking unit after back surface printing performed on the sheet having undergone front surface printing is later than feeding timing of a target paper which is to be precedingly subjected to back surface printing (referred to as “back surface printing target sheet”). For this reason, in a conveying layout in which a sheet arrival time from the feeding position (the sheet stacking unit) of the front surface printing target sheet to the transfer position is longer than a sheet arrival time from the feeding position of the back surface printing target sheet to the transfer position, the productivity may decrease.
In recent years, there has been developed an image forming apparatus that prevents a decrease in productivity at the time of duplex printing even when a distance from the feeding position of the front surface printing target sheet to the transfer position is lengthy. For example, according to a technique disclosed in Japanese Patent Application Laid-open No. 2002-096977, paper feeding timing is managed in view of both a transfer order and a paper feeding order; in the interleaf control at the time of duplex printing, when the distance between the sheet stacking unit, from which the front surface printing target sheet is newly fed after back surface printing is performed on the sheet having undergone front surface printing, and the transfer position is lengthy, a paper feeding order changes, so that the sheet is fed from the sheet stacking unit prior to the back surface printing target sheet.
However, in the technique disclosed in Japanese Patent Application Laid-open No. 2002-096977, it has been difficult to resolve the problem in that productivity decreases at the time of duplex printing when the interleaf control is not performed. Further, in the technique disclosed in Japanese Patent Application Laid-open No. 2002-096977, as long as there is a printing request, if a condition such as a distance or paper feeding from a certain sheet stacking unit is satisfied, the front surface printing target sheet is precedingly fed. Thus, there may be fed sheets which are more than the number of sheets necessary for satisfying prescribed productivity. For this reason, software control for sheet control becomes complicated, and thus the consumption of a memory resource such as a random access memory (RAM) used for the software control may increase.
Further, it is necessary to guarantee a printing order or a sheet interval between the front surface printing target sheet which is precedingly fed and the preceding back surface printing target sheet. That is, when timing of the front surface printing target sheet fed from the sheet stacking unit is not adjusted anywhere, the front surface printing target sheet may pass the preceding back surface printing target sheet or may bump into the preceding back surface printing target sheet, leading to paper jamming.